1. Field of the Invention
The present invention is generally concerned with non-geostationary satellite constellations in telecommunications systems with satellite-fixed cells.
2. Description of the Prior Art
Several non-geostationary satellite constellation projects propose to offer circuit-mode telecommunication services such as telephony, facsimile and transmission of data to mobile users. These project include the IRIDIUM (registered trademark) project of the American company MOTOROLA, the GLOBALSTAR (registered trademark) project of the American company LORAL SPACE SYSTEMS, the ICO project proposed by INMARSAT and the ODYSSEY (registered trademark) project of the American company TRW.
The satellites in these constellations are equipped with multibeam antennas. Each beam defines a contour on the surface of the Earth delimiting a coverage area called a "cell". With no specific device for varying with time the pointing of the antennas, the beams and therefore the associated cells remain in a fixed position relative to the body of the satellite. This concept is therefore known as the satellite-fixed cell system. Because of the movement of the satellites, the cells move over the surface of the Earth and calls in progress have to change cell if the duration of the call exceeds the time for the cell to pass over the user. This change of cell is known as handover from beam to beam for a call, i.e. "beam handover". It may be necessary for the calls to be transferred from one satellite to the next, which is known as "satellite handover".
The cell or the satellite taking over the call in progress may not have any free channel at the time of such handover and the call is then interrupted, which is known as "forced call termination". Mobile phone users currently served by terrestrial cellular networks encounter the same problem, but here because of the mobility of the users. For fixed users of terrestrial cellular networks who remain in the same cell throughout the duration of their call, there is no handover of the call from one cell to another, and therefore no interruption of the call. Given that the non-geostationary satellite constellations are intended to constitute an extension of the terrestrial cellular service, it would be desirable to be able to offer a service quality equivalent to that of terrestrial cellular networks. Furthermore, these constellations could provide communication services to fixed users. To offer fixed users a service quality equivalent to that of terrestrial systems or of geostationary satellite systems, interruption of calls due to cell-to-cell handovers resulting from the movement of the satellites must be avoided.
The non-geostationary satellite constellations envisaged at this time are intended to offer communication services to mobile users and could offer services to fixed users but without guaranteeing mobile or fixed users successful beam handovers or successful satellite handovers. This means that the service quality offered to fixed users in such constellations would be worse than that provided by other telecommunication systems such as with microwave beams, optic fibers, geostationary satellites, etc. This drawback means that such constellations are not a very beneficial alternative for fixed users.
One method of guaranteeing call continuity to users on beam handovers or satellite handovers in non-geostationary satellite constellation systems is to reserve a channel in each of the cells that the user will pass through during the call when a user submits a call set-up request. Apart from the fact that it is difficult to predict the exact duration of a call, and therefore the number of cells concerned, this method leads to unnecessary over-rating of the capacity of the satellite.
In another method, disclosed in the article by the inventors entitled "Providing appropriate service quality to fixed and mobile users in a non-geo satellite-fixed cell system" published in "Proceedings of the Second European Workshop on Mobile/Personal Satcoms" EMPS'96, Part 2, pages 79-96, immediately after a call set-up request a user reserves a channel in the cell in which he is located and in the next cell that he will have to cross. Thereafter, each time user enters a cell, he requests reservation of a channel in the next cell. This method guarantees the transfer of user's calls from one cell to the other, but only in the particular case in which all the cells are identical, rectangular in shape, contiguous and aligned relative to the direction of movement of the satellites.